I too am having this problem. I think those who aren't having this problem have established mail servers, with good reputation related to their public IPs, so Gmail and Hotmail let their messages through even though it has the localhost and 127.0.0.1 in the headers.

We recently setup a new server with a new IP and domain. The IP is clean and passes all the spam database checks, however it doesn't have rep.
I've ran all the mail tests I could find on several sites, including mxtoolbox, and the server passes with 0 problems.
On Email Security Grader - Test your mail server it scored:

All email, even standard plain text emails being sent from the new server are being sent to the spam folders at Gmail and Hotmail.
Gmail's reason is: "Why is this message in Spam? It's similar to messages that were detected by our spam filters."
Further reading here for that specific reason:https://support.google.com/mail/answ...=mail&expand=5
"Messages sent from accounts or IP addresses that have sent other spam messages"

How much spam has been sent with the IP 127.0.0.1 in the headers? I imagine a lot.
Having localhost and 127.0.0.1 in the headers is the only suspicious thing I can find. I found another thread where instructions were given on how to remove them on previous versions of Zimbra, however I was not able to get it to work on Zimbra 8:http://www.zimbra.com/forums/adminis...headers-5.html